For a number of years there has been a growing appreciation of the danger and other disadvantages of the common practice of fixing a rotary mower blade directly on the depending drive shaftof an engine. U.S. Pat. No. 3,026,665 for example, points out that such direct drive causes engine shafts to be bent when the blade strikes an obstruction, that overloading the direct-driven blade stalls the engine, with resulting inconvenience, and that that direct drive endangers the operator and others because the blade always rotates with the engine, even during starting and idling. The seriousness of this latter danger now appears to be leading to government safety requirements for a "deadman" lever on the mower handle which will stop the blade except when the operator is in operating position and holding the lever. To be effective, a deadman control requires high reliability, and if its use is not to impose undue cost on the consumer, a highly practical, effective, and inexpensive mounting and control arrangement is required. Consequently, there is a need to provide such a blade mounting and control arrangement which can be applied to various presently-designed mowers with a minimum of change and expense.
Certain structures have been designed with the intent to de-clutch and decelerate a lawn mower blade which is co-axially mounted to the shaft of an internal combustion engine. One approach taken, however, has been to utilize a centrifugal clutch for example of the type shown in U.S. Pat. No. 4,035,994. To the outside of such clutch drum is mounted a hand brake which is applied by spring tension and released by operator actuation of the deadman control which also simultaneously causes an increase in engine speed and thus causes the clutch to be engaged.
The problem with a centrifugal clutch approach to this problem is that it tends to be self applying and in that sense is not a fail safe device. For example, if the spring that is to apply the brake were to either break or lose tension, the operator might release the deadman control and have the blade continue to drive from the engine. The operator could be unaware that the blade had failed to stop.
Power lawn mowers are manufactured and distributed in very large numbers in the consumer market. Maintenance must be considered uncertain, at best. It is a statistical certainty that substantial numbers of mowers will be operated by the public with mechanical defects. It is therefore obvious that a clutch-brake mechanism which is intended as a safety device should be so designed that a mechanical defect in any component of the mechanism will tend to result in the blade being disconnected from the engine, rather than the reverse. It is believed by many that a safety control that is not fail-safe in operation is worse than no control at all.